Windows including glass substrates with solar control coatings provided thereon are known in the art. Such windows may be used in the context of architectural windows, insulating glass (IG) window units, automotive windows, and/or the like.
Commonly owned U.S. Pat. No. 5,837,108 discloses a heat treatable coated article having a layer stack of glass/Si3N4/NiCr/Si3N4. In this coated article, the silicon nitride overcoat is the sole protector of the NiCr infrared (IR) reflecting layer. The silicon nitride overcoat is for protecting the NiCr layer from both mechanical damage (e.g., scratching) and chemical attacks.
Unfortunately, it has recently been discovered that when coated articles such as this (i.e., glass/Si3N4/NiCr/Si3N4) are heat treated (e.g., thermally tempered, heat strengthened, or the like), tiny pinholes tend to form in the silicon nitride overcoat layer due to such heat treatment. After the pinholes form in the silicon nitride overcoat due to heat treatment, the silicon nitride overcoat while still being a good mechanical protector is less than desirable with respect to chemical protection. In other words, the coated article (especially the NiCr layer) is subject to corrosion. Thus, heat treated coated articles with an overcoat of merely silicon nitride are sometimes lacking with respect to chemical durability, both in the field and/or during transport/storage when exposure to certain chemicals can occur.
Other known coated articles use a protective overcoat of tin oxide. Unfortunately, tin oxide overcoats are undesirable with respect to mechanical durability (e.g., scratch resistance).
DE 3027256 discloses a coated article having dual-layer structure which deals with TiO2 and TiN/TiOxNy. Unfortunately, these materials are too porous and lead to a lack of durability in commercial applications.
Still further coated articles use a dual layer protective overcoat of silicon nitride and tin oxide. For example, see parent published application Ser. No. 2002/0064662 (see also corresponding U.S. Pat. No. 6,576,349) which is incorporated herein by reference, which discusses a protective overcoat having an outer layer of silicon nitride and an underlayer of tin oxide. Unfortunately, while the silicon nitride provides good protection against mechanical damage (e.g., scratching), after the tiny pinholes are formed in the silicon nitride due to heat treatment as discussed above, the underlying tin oxide often does not provide sufficient protection with respect to chemical attacks on the coating. In other words, the tin oxide portion of the tin oxide/silicon nitride/air dual layer overcoat of Ser. No. 2002/0064662 merely acts as an optical matching layer as opposed to a protector against chemical attack. As a result, it has unfortunately been found that despite the presence of a dual layer overcoat of tin oxide/silicon nitride/air, such heat treated products may sometimes suffer from high temperature/high humidity failures when subjected to certain chemicals.
In view of the above, it can be seen that there exists a need in the art for an improved overcoat for heat treatable coated articles. In particular, there exists a need in the art for an overcoat that is more chemically durable.